debugging calculated value javaScript

Read:

• https://jazz.net/library/article/1003
  
• https://jazz.net/wiki/bin/view/Main/AttributeCustomization
• Process Enactment Workshop for the Rational solution for Collaborative Lifecycle Management
  Lab 5
• https://jazz.net/library/article/1360

This is an example for how to compare dates:

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dojo.provide("org.example.DateValidator_30_days_later");

dojo.require("com.ibm.team.workitem.api.common.WorkItemAttributes"); // To access the work item attributes
dojo.require("com.ibm.team.workitem.api.common.Status"); // To create a status object to return
dojo.require("com.ibm.team.workitem.api.common.Severity"); // To create a severity for the status
dojo.require("dojo.date"); // We need the date class from Dojo to compare two dates
dojo.require("dojo.date.stamp"); // We need the stamp class to work with ISO date strings
(function() {
var Severity = com.ibm.team.workitem.api.common.Severity;
var Status = com.ibm.team.workitem.api.common.Status;
var DateValidator = dojo.declare("org.example.DateValidator_30_days_later", null, {
validate: function(attributeId, workItem, configuration) {

    var severity= "ERROR"; 
    var message= "Date must be at least 30 days from now";

   // Work Item new: not initialized - provides todays date. Once created provides the creation date 
    var beginDate = dojo.date.stamp.fromISOString(workItem.getValue("com.ibm.team.workitem.attribute.creationdate")); 
    // Get the current attribute's value and make a Date object from it
    console.log("I got: ["+workItem.getValue(attributeId)+"]");
    var endDate= dojo.date.stamp.fromISOString(workItem.getValue(attributeId));        
    // Compare the two dates returns negative value due to order
    var compare = -(dojo.date.difference(endDate, beginDate, "day"));
    if (compare>= 30) { // make sure endDate is not earlier than beginDate + 30
        return Status.OK_STATUS;
    } else {
        return new Status(Severity[severity], message + " current value: [" +endDate + "] and difference is " + compare);
    }
}

});
})();

 You should Gooogle for JavaScript Funktion to calculate with dates and read the documentation about attribute customization in the team wiki. There is also a process enactment workshop in the library you can download and read to get at this information. You have to do a conversion to correctly read and write dates which is described in the attribute customization API and plus and minus is not needed to calculate date differences.

I can see nothing wrong in your code. All you are working on are just numbers, and there is nothing built-in to differentiate between any data and "now". If you are saying you get 2 instead 4, I suspect there is something wrong outside of the code (well, if you simply retrieve the data from a wrong attribute, only you can tell). Adding console.log() in your code will make it easy for you to see.
https://www.w3schools.com/js/js_debugging.asp